Method for producing a magnetic device

ABSTRACT

A magnetic device and method for making it involve a magnetic device specifically constructed of grains in a matrix. The matrix may be cement or plaster. The grains have an average diameter that is greater than their magnetic domains. The device may be applied to shielding applications for frequencies ranging from 100 kHz to 10 GHz. The shielding may be applied to walls of a building, consumer products such as magnetic disks, and the like. The grains may be any ferromagnetic material, including ferrite.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of U.S. application Ser. No. 09/756,530,filed Jan. 10, 2002, which was a continuation of Internationalapplication number PCT/DE99/01950, filed Jul. 1, 1999, which designatedthe United States, and which was not published in English.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for producing amagnetic device in environments where, for example it is desired toprovide for or against the passage of electromagnetic radiation. Suchapplications may range from a shielding for a cable to a plastering fora wall of a room or rooms. While the present invention will be describedwith respect to the latter example, applications of the presentinvention is limited only by the imagination of one skilled in the art.The present invention relates particularly to magnetic material grainsize. The related art sets out two examples of consideration of grainsize in production and applications.

[0004] Reference WO 92/08678 A1 concerns a magnetized product formed bythe hardening of cement and water into a malleable mass. Such masscontains granular magnetic materials, including soft iron, therebydisplaying certain magnetic properties. The material may be applied inelectrical dynamos.

[0005] Reference EP 0393599 B1 sets out a surface mounted electricalcoil which may be surrounded by a product of synthetic material whereina powdery magnetic material such as carbonyl iron or ferrite isdeposited.

SUMMARY OF THE INVENTION:

[0006] Ferromagnetism refers to the physical phenomenon wherein certainelectrically uncharged materials strongly attract others. Ferromagneticmaterials, when exposed to magnetic fields, become easily magnetized andotherwise display strong magnetic properties. The magnetism inferromagnetic materials is caused by the alignment patterns of thematerials constituent atoms, which act as elementary electromagnets.Ferromagnetic material may be considered as comprising a large number ofsmaller regions, or domains, wherein the atomic or ionic magneticmoments are aligned. Where the combined moments of the domains arerandomly oriented, the ferromagnetic material will not display anysignificant magnetic properties. However, when an external magneticfield is applied, the moments will rotate into alignment with the field,thereby reinforcing one another and causing magnetic properties of thematerial as a whole to be displayed. Such alignment continues until apoint of saturation, particular to individual ferromagnetic materials.Examples of such materials includes, iron, cobalt, nickel, alloys orcompounds containing each, along with some rare earth metals, as knownto one skilled in the art.

[0007] Ferrite is a ceramic-like ferromagnetic material having a varietyof applications. A determining factor of ferrite grain size, in a givenmaterial, is its initial permeability which refers to the initial slopeof the magnetic flux density (8), established within the material by amagnetizing field, versus the magnetic field strength (H) of themagnetizing field. The slope is typically characteristic of anunmagnetized ferromagnetic material and otherwise represents themagnetic permeability under very small-applied magnetic fields. Magneticpermeability refers to the relative increase or decrease in theresultant magnetic field inside a particular material as compared withthe magnetizing field wherein the material may be located. Magneticpermeability is typically assigned the Greek character mu and defined asB/H.

[0008] Returning to ferrite, a determination of essential grain size isthe initial permeability. The initial permeability of a solid includingferrite therein depends on the ferrite grain size diameter and itsrelative size with respect to domains of the solid material. A typicaldomain has a diameter of about 0.5 microns. Where the average graindiameter size of ferrite approaches the size of the domain, initialpermeability is reduced. The reduction occurs because the number ofdomains per grain dramatically drops, such that the alignment of thedomains in response to an applied magnetic field discontinues.

[0009] It is therefore an object of the invention to set out a methodfor making it of ferromagnetic material having an elevated or maximizedinitial permeability. In particular, the material is applicable toshielding for frequencies of above 100 kHz, and in particular 1 MHz to 2GHz. It is another object of the invention to provide a method which canbe implemented so as to enable mass production at reasonable engineeringeffort and expense and with maximally replicable componentcharacteristics.

[0010] The present invention comprises a magnetic device comprising aplurality of ferromagnetic material grains, said grains having a minimumdiameter larger than an average domain diameter of said grains, and saidgrains further embedded in a matrix. The present invention furthercomprises a method for producing a magnetic device, comprising the stepsof: forming ferromagnetic grains, said grains having an average diametergreater than domains of said grains; and embedding said grains in amatrix, said matrix comprising a hardening material.

[0011] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0012] Although the invention is illustrated and described herein asembodied in a method for producing a magnetic device, it is neverthelessnot intended to be limited to the details shown, since variousmodifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

[0013] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a sectional view of the present invention as applied toplaster for a wall;

[0015]FIG. 2 is an enlarged view of a portion of FIG. 1;

[0016]FIG. 3 is a sectional view of the present invention as applied toshielding for a magnetic disk

[0017]FIG. 4 is a cross section of the magnetic disk depicted in FIG. 3

[0018]FIG. 5 is a diagram depicting permeability of materials applied inaccordance with the present invention; and

[0019]FIG. 6 is a second diagram depicting permeability materialsapplied in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The following sets out an embodiment of the present invention,wherein like reference numerals refer to equivalent functioningelements. FIGS. 1 and 2 depict a similar embodiment of the presentinvention, namely, application of the invention in a plaster forcovering a wall. As shown in FIG. 1, a wall 1 is covered with a plaster2. The plaster, as will be detailed below, is a shielding againstparticular electromagnetic frequencies common to moderntelecommunications, namely 100 kHz and above. The wall itself may be ofany suitable construction envisioned by the skilled artisan, includinglayers of bricks, cement, and the like. The plaster 2 comprises ashielding and contains grains of ferrite embedded in a matrix. Theferrite may be sintered, iron and the like. FIG. 2 sets out a portion ofthe shielding, II, in more detail. As shown therein, the shield 2,comprises a plurality of ferrite grains 3, 4 arranged in a matrix 5 ofhardened cement. The matrix may comprise equivalent materials envisionedby the skilled artisan to effectively accommodate a ferromagneticmaterial, such as ferrite, in a shielding matrix formation. The depictedgrains 3, 4 have essentially larger diameters than a typical domaindiameter of ferrite, namely 0.5 microns. As such, a relatively highinitial permeability for the shielding 2 is effected. The size of thegrains 3, 4 may range up to 10 millimeters thereby effecting thedepicted non-homogeneous shield 2. The grains may have a minimumdiameter of about 10 microns and an overall total average diameter ofabout 100 microns. The shield may comprise 5 parts cement, 5 partswater, and 100 parts grains. Hence the shield may have at least 80%grain density, and preferably about 90-95% grain density.

[0021] In the depicted embodiment, the shielding 2 includes a firstnumber of grains 3 and a second number of grains 4. The first number ofgrains has an average diameter of about 8 mm and the second number ofgrains includes an average diameter of about 2 mm. Taken as a group, thediscrepancy among the diameters can be relatively high, as for example,for grains 3, the range may be between about 1 mm and 12 mm and forgrains 4, the range may be between about 0.1 mm and 6 mm. The depictedmatrix may be constructed of cement, as may be manufactured by AALBOGPORTLAND A/S of Aalborg, Denmark and may include fine-granular componentmaterial such as silizimudioxide, aluminum oxide, calcium and otherrelated materials as well as a surface active medium to elevate the flowability of the cement ferrite matrix mass. An advantageous arrangementincludes using cement, which requires relatively little water, ascompared with other available cements, so as to avoid pores within thematrix 5.

[0022]FIGS. 3 and 4 depict a magnetic disk manufactured according to thepresent invention. Herein the disk, preferably a magnetic disk, includesan electromagnetic field with the above frequencies (above 100 kHz) andmanufactured with the equivalent materials discussed above with respectto FIGS. 1 and 2. Other applications may include a coil, circuit and thelike.

[0023]FIG. 5 depicts the initial permeability of a product builtaccording to the above specifications versus the percentage or densityof ferrite. The abscissa refers to permeability while the ordinaterefers to the ferrite percentage. FIG. 6 depicts the interdependence ofpermeability with frequency of a magnetic field impinging upon a solid.The permeability is in essence a complex valence of the influence of themagnetic field upon the solid. The real component of the graphedpermeability, as depicted in FIG. 5, is in effect a proportion ofmagnetic flux density B and magnetic field strength H as impinging uponthe solid. The imaginary component of the graphed permeability sets outdampening or absorption of the magnetic field as applied to the product.Where such loss becomes substantial, a dispersion effect is made uponthe magnetic field.

[0024] Continuing with FIG. 6, here the effects on a magnetic field oftwo products of slight composition variation, is depicted. Inparticular, the imaginary and real components of the permeabilitymeasurements discussed above. Like letters refer to like products. Theabscissa represents the real and imaginary components of permeabilityand the ordinate represents a logarithmic scale of frequency. The curvesnot beginning at the origin refer to the real component of permeability,while the curves beginning at the original refer to the imaginarycomponent. As depicted, at lower frequencies, the real components arepractically constant, while the imaginary components are relativelynon-existent. At frequencies about 10 MHz, the real components begin toascend while the imaginary components climb. As such, about 100 MHz, theimpinging magnetic field essentially becomes dispersed. Hence, productsconstructed with the above ferrite density and size effectively shieldat about 100 MHz. Such a shielding is particularly useful in thetelecommunication arts as the frequencies lie in the pulse range ofconventional microprocessors as well as conventional mobile radio units.

[0025] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications would be obvious to one skilled in the art are intended tobe included within the scope of the following claims.

We claim:
 1. A method for producing a magnetic device, comprising thesteps of: forming ferromagnetic grains, said grains having an averagediameter greater than domains of said grains; and embedding said grainsin a matrix, said matrix comprising a hardening material.
 2. The methodaccording to claim 1, wherein said ferromagnetic grains compriseferrite.
 3. The method according to claim 2, wherein said ferrite issintered.
 4. The method according to claim 1, wherein said hardeningmaterial comprises cement.
 5. The method according to claim 4, whereinsaid hardening material comprises about 5 parts cement, 5 parts waterand 100 parts grains.
 6. The method according to claim 1, wherein saidmagnetic device is a shield for frequencies in the range ofapproximately 100 kHz to 10 GHz.
 7. The method according to claim 1,wherein said grains have a minimum of about 10 microns.
 8. The methodaccording to claim 1, wherein said grains have an average diameter ofabout 100 microns.
 9. The method according to claim 8, wherein saidgrains comprise first and second groups of grains, said first grouphaving an average diameter of about 8 millimeters and said second grouphaving an average diameter of about 2 millimeters.
 10. The methodaccording to claim 1, wherein said hardening material is a plaster. 11.The method according to claim 1, wherein said magnetic device is a shellfor an electromagnetic module.
 12. The method according to claim 11,wherein said module is a coil.
 13. The method according to claim 11,wherein said module is a circuit.
 14. The method according to claim 11,wherein said module is a magnetic disk.